Forgive me if this information lies already somewhere. I am a Sigil fan but I like the calibre Editor because it provides some additional and very nice features. OTOH some features are also missing in the Editor. Today, I begin to modify my EPUB with Sigil and then end it and validate it with the Editor. Sigil then the Editor, in this particular order as I had been advised.

When calibre's Editor will be able to do everything, I'll totally switch to it.

Missing features

For me, the following three are the most important:

1- Metadata editor
One exists in calibre, but not in the Editor. A basic tool but quite useful.

2- Saved clips and searches.
This is a very handy Sigil feature that I really need everyday several times.

3- Spellchecker
Sigil has a very nice spellchecker, precious for proper nouns (it gives the number of occurrences of every mispelled name and can deal with typographical apostrophe).

My question is: do these missing features are planned to be implemented with calibre's Editor? The question of When? comes also to mind, but this is in fact secundary.

Well, sometime in the last -- day I think? -- Kovid created a new development branch called "spell" which contains the beginnings of dictionary implementation. So I think we will see this pretty soon.

Metadata editing can already be done in the GUI, and calibre will update the metadata if you open the book from the library. Not sure if/when that will be added into the editor core. Clips are planned, but I'm not sure when.